My son came home with instructions from his Music Teacher at school to create his own instrument, so I encouraged him to come up with a plan and get to work.
He's been interested in our old GameCube version of Legend of Zelda: The Wind Waker (thank goodness it still plays on our Wii -- it's our favorite!), so he wanted to make a harp.
It's not the loudest instrument in the world, but it is his own invention.
I think it's a wonderful project for any kid. We'll have to try it again in the future!